1. Field of the Invention
The present invention relates to an organic field-effect transistor provided with an organic semiconductor layer and a gate insulating layer. It also relates to a production method and an intermediate structure (structural body) for such an organic field-effect transistor as well as to an organic field-effect device. More particularly, the present invention relates to an organic field-effect transistor that operates at a low drive voltage and yet assures a sufficiently high current gain and a high-speed response. The transistor according to the present invention can be used as an amplifying element that amplifies current flowing between source and drain electrodes or a switching element that turns ON/OFF the flow of such current.
2. Description of Related Art
A field-effect transistor includes a source electrode, a drain electrode, a semiconductor layer in contact with the source and drain electrodes, a gate insulating layer adjacent to the semiconductor layer, and a gate electrode in contact with the gate insulating layer.
In recent years, extensive studies have been conducted on organic field-effect transistors with organic semiconductors instead of inorganic semiconductors and the gate insulating layers thereof constituted with electrolytes instead of dielectric materials. Organic transistors are advantageous since they are not only light in weight but are also thin and flexible so that they can be bent. Accordingly, the organic transistors are expected to expand applications of transistors.
Also, studies have been conducted on a polymer gel electrolyte comprised of polymer gel (polyethylene glycol) and Li ions (see. for example, J. Takeya et al., Appl. Phys. Lett. 88, 112102 (2006)) (hereafter referred to as “Polymer Gel Electrolyte I”). The polymer gel serves as glue or thickener. The Polymer Gel Electrolyte I is not liquid but paste-like.
It has been observed that the application of a voltage to the Polymer Gel Electrolyte I causes ions to move, resulting in the formation of a layer in which positive ions are accumulated and a layer in which negative ions are accumulated over a region having a thickness of about 1 nm from the electrodes which, together, form layers in which the balance between positive and negative charges is lost (electric-double layers). In these layers, the electric field is concentrated in a region with a thickness of about 1 nm from the surface of the semiconductor. As a result, even when a voltage as low as 1 V is applied between the gate electrode and the source electrode (or the drain electrode), an electric field as strong as 10 MV/cm is applied in the layers. Accordingly, when the Polymer Gel Electrolyte I is used as the gate insulating layer of a transistor, a stronger electric field can be applied to it at a lower voltage. As a result, many carriers are injected into the gate insulating layer. A high current gain can be achieved even at a low drive voltage in such a transistor.
In the meantime, there has been proposed an organic field-effect transistor with a gate insulating layer made of a polymer gel electrolyte comprised of a polymer gel and an ionic liquid (for example, 1-butyl-3-methylimidazolium hexafluorophosphate) (hereafter, referred to as “Polymer Gel Electrolyte II”) (Jiyoul Lee et al., “Ion Gel Gated Polymer Thin-Film Transistor” J. Am. Chem. Soc. 129 (2007) 4532), instead of L1 ion.